USPS is US Parcel Service, the equivalent of our Post Office, are not a door-to-door service, and they take quite a while to get through. My experience is that if your seller used the International service, then it could take up to four weeks to get here. USPS say clearly that the tracking number is only inside the US, and that usually means 'collected' since they don't track overseas. The parcel is basically sent to the UK Post Office for delivery to you.
UPS is a different ball game, as they have an international service door-to-door including customs clearance. They take anything from a few days to something under two weeks, depending on how long it takes to clear customs.
Expect to pay VAT and duty, and also an admin fee from the Post Office for doing it all for you - although you don't get the choice to do it yourself! That's one reason it's cheap for the seller, whereas the UPS cost includes all the admin for clearing through customs and is more expensive for the seller. If you have the choice in the future, make the seller end UPS, not USPS.
Once you get the parcel and have to pay the bill, check it. Mark Christopher posted on here that he had been overcharged for VAT, and I think he said he got it reduced when he challenged it. You should pay VAT on what they value the package at and the duty - tax on a tax!!
Patience is a virtue, but do check with the seller when they have to lodge a claim for a lost parcel. If you wait too long then it is likely you will have gone outside the claim cut-off date. HTH
PS - making a false declaration on the value of the packet is a criminal offence, and the declarer is liable to go to court. Some do, most won't.